MICROSCOPIC GARDENING. 



21 



results to horticulture and agriculture, obtained by microscopic 

 gardeners, are those which have led to our modern suggestions 

 for the practical treatment of diseases of plants. 



So long as people believed, with Unger, that a parasitic 

 fungus was merely a diseased exudation of the plant itself, no 

 proper treatment could be thought of, and long afterwards, while 

 men only knew that a parasitic disease was at work by the 

 extrusion of the fungus-spores, it was impossible to attack the 

 matter successfully, because the existence of the disease was not 

 recognised till it had all but run its course — it was like calling 

 in the doctor when we recognised the patient was dying. 



As soon as the proof was forthcoming that a parasitic fungus 

 has a determinable life -history, partly outside and partly inside 

 the plant, however, it became clear that, provided we can catch 

 the spores or fungus outside the plant, it ought to be possible to 

 attack them. Even the earliest discoveries of De Bary and his 

 contemporaries showed that this hope was not a vain one. 



When it became clear that the " smut " of corn, the fungus 

 of wheat "rust," and the " mildews " of the rose, vine, hop, 

 and potato, only dwell for a few weeks in the tissue of the host, 

 and that before they establish themselves in these tissues the 

 spores have to pass through a period of germination, in which 

 their delicate germ-tubes are so exceedingly sensitive to external 

 agents that the merest trace of acids or alcalies, or poisons of 

 various kinds, kills them in a moment, it seemed obvious that 

 all we had to do was to apply a selected "weed-killer" to the 

 germinating spores. 



Now you all know how comparatively easy it is to kill weeds 

 on a gravel path, where no considerations for other plants need 

 affect our calculations as to the kind of poison used, or the 

 quantities and strengths employed ; well, just in the same way 

 the microscopic gardener, by applying antiseptics to his pure 

 cultures, soon found that a mere trace of sulphurous gases, 

 carbolic acid, copper-salts, corrosive sublimate and so on, at once 

 destroys the baby fungus -plant as it emerges from the spore. 



But when it came to applying such poisons to the spores 

 germinating on the host-plant — i.e. on a rose, vine, hop, &c. — 

 matters were complicated by the very biological conditions which 

 render it much more hazardous to attempt any poisoning treat- 

 ment on weeds in a lawn or in a flower-bed — viz. you run the 



